The Heathkit AD-1702 is an active low-level crossover ideal for adding subwoofers to any stereo playback system. It's fed from your preamp, and has outputs to feed to the power amplifiers for both your high and low speakers.

The Heathkit AD-1702 crossover is unusually handy because it provides separate adjustment of frequency and slope for each of the high pass and low pass sections. For example, with a normal sealed-box speaker, set the high-pass crossover to 6 dB/octave at the speaker's -3 dB rolloff point, which makes the combined acoustic response of the regular speaker 18 dB/octave. Set the subwoofer's crossover to 18 dB/octave at the same frequency, and you've got perfect acoustic response for the complete system, taking the speakers' own responses into account.

It's a stereo crossover, ideally intended for stereo subwoofers. Even though our ears can't localize a mono woofer crossed-in at 60 Hz or below at 18 dB/octave, we can localize woofers crossed over at higher frequencies or with shallower slopes. Most importantly, music and movies on CD, DVDs and better has stereo bass. You can slum it with a mono woofer, but if you want serious high fidelity for music, unless you only listen to LPs and 78s, you need stereo subwoofers. CDs and movie DVDs have always had stereo bass. See more at stereo subwoofers.

You feed the low outputs either to your own amp and passive subwoofers (I used JBL B460 18" subwoofers fed from a Hafler DH-200), or feed the crossover's outputs to the RCA inputs of today's popular self-powered subwoofers.

It has a hard-wired bypass, a level control for the low output, separate mute controls for each of the high and low outputs. The level control knob is on a splined, not set-screwed, shaft, so if you want to index the knob to zero for a certain gain, it only goes in steps.

The only thing missing is a polarity switch for the low outputs.

The "Bass Overload" LED lights whenever either low output exceeds about 1 RMS. The crossover still has 20 dB of headroom at 1 V RMS; this light is letting you know that most power amplifiers are going to be at about maximum output at 1 V RMS input.

Yes, 5.1 has only one mono low-frequency effects (LFE) channel, however the other 5 channels are full-range, and contain all sorts of multichannel low-frequency effects if you'd only listen to them. 5.1 is 5 point 1, not (5 x 0.9) point 1. See also Bass Management.

If you settle for a mono subwoofer, the AD-1702 also has a summed mono woofer output. It thirdly has a Right and (minus) Left pair of outputs as a sub-optimal attempt to feed any stereo amp as a high-power mono subwoofer amp. Ideally, one ought to redesign it so that pair of outputs is R+L and -(R+L).

It's in a steel sheet metal case, covers held by sheet metal screws. The board hangs upside down: removing top cover reveals the bottom of the board. Remove the bottom cover to see the top of the board and the components

Relays provide about an 8 second turn-on delay. The relay shorts the outputs to ground after the 560 ohm build out resistor. I measure 20 seconds delay in my factory-built unit.

In July 2014, at 11.0V RMS output from the bass section, adding gain from the 10V output of the UPL, I measured 0.4% THD.

The Bass Overload light is just a guide to overdriving your subwoofers, not the cross over itself. It lights at about 50% brightness at 750mV RMS output from either LF section, and gets brighter until it's 100% bright at 1.5V RMS out from either LF section.

One huge gotcha of buying these used today is that some people have changed parts to change the crossover frequencies. If someone did, this makes the crossover useless until you change the parts back to the correct values, or at least to the values correct for your use.

The best way to test is by plotting frequency response with an audio analyzer, but barring that, look at the capacitors on the lower left, flanking each of the frequency selector switches:

After the original owner has changed this, normally no one knows until the next unhappy buyer — and only if that buyer has an audio analyzer to figure out what's going on. The bypass switch is often disabled if the crossover was scaled for use with tweeters.

Amazingly, I've seen listings on eBay with sufficiently detailed interior shots to let me see that a crossover had been scaled, saving me the trouble of buying it. (my technician charges too much to reswap capacitors.)

To scale or rescale a unit, you'll need at least the skills of a licensed technician, and the original manual with the values and locations of the capacitors.

10x frequency-scaled sample

Measurements of a simple with both the HF and LF sections scaled by 10x. In other words, the "100 Hz" position was really a 1 kHz crossover.

It's great for use in any bi-amped system. I used one for years to use a pair or 8-cubic-foot 18" B460 (JBL 2245H) subwoofers with a pair of Quad ESL-63s. This crossover let me cross them over at 40 Hz, which lets the Quads sing and the JBLs do nothing other than move air.

The bridged outputs are done incorrectly: they aren't an out-of-polarity mono sum; instead, they are the right output and the inverted left output.

These were all sold as do-it-yourself kits, so there is always the possibility that you might get one today assembled by an idiot.

WARNING: Of three I've seen for sale, all three were frequency-scaled, meaning that the prior owner had severely altered the crossover frequencies by changing internal capacitor values, but had not noted this while selling these.

One tip-off is to look for green Mylar capacitors near the frequency and slope switches in any online photos, which were the original capacitors. Different capacitors suggest different frequencies, especially if you see any rectangular or cylindrical polypropylene capacitors, which were not standard.

I have no idea why people felt the need to scale these by 10x, especially since that's a relatively useless frequency shift, but was the one outlined as an example in the manual. If you buy one that's been shifted, simply changing capacitors can change it back.

I support my growing family through this website, as crazy as it might seem.

The biggest help is when you use any of these links to Adorama, Amazon, eBay, B&H, Ritz, Calumet, J&R and ScanCafe when you get anything, regardless of the country in which you live. It costs you nothing, and is this site's, and thus my family's, biggest source of support. These places have the best prices and service, which is why I've used them since before this website existed. I recommend them all personally.

If you find this
page as helpful as a book you might have had to buy or a workshop you may
have had to take, feel free to help me continue helping everyone.

If you've gotten your gear through one of my links or helped otherwise, you're family. It's great people like you who allow me to keep adding to this site full-time. Thanks!

As this page is copyrighted and formally registered, it is unlawful to make copies, especially in the form of printouts for personal use. If you wish to make a printout for personal use, you are granted one-time permission only if you PayPal me $5.00 per printout or part thereof. Thank you!